It is an issue with your local Steam. Sadly Steam is unhelpful with their k_EResultFail as it's their generic "There is an issue" message.

So it's not firewall, nor their EULA.

1) Have you got a lot of save files / workshop subscription ? (If so try to delete a bunch and see if it helps)

2) Does the workshop works for other games by any chance ?

3) If the firewall did not help, there is still a chance that your router is blocking the traffic, if you have any chance of uploading a map outside of your normal environment and it works, that would point to the router. (like this thread for example)

I'm running out of other idea to find the culprit. But the map uploading fine on my side shows that something somewhere in your local setup prevents it ^^' We just need to find out what is preventing the upload